Reporting in the March 21 issue of the journal Nature, an international team of researchers says the skull provides yet another piece of evidence that a single human ancestor, Homo erectus, ranged across Europe, Asia, and Africa as long ago as 1.8 million years.

For the last two decades, the question of whether fossils discovered from between two million and one million years ago represent one species or numerous branches on the family tree, some of which died out, has been a hot button of debate.

Tim White, a paleoanthropologist at the University of California, Berkeley and co-author of the study in Nature, believes the partial skull found in Ethiopia resolves that question. "The matter of early hominid distribution and species count is solvedone [species] at a million [years], from Spain to China to Java to Africa," he said.

The skull, he said, represents an evolutionary intermediate step linking older, more primitive forms of the species with younger, more human-like forms.

Other experts, however, disagree with that conclusion, and the issue remains controversial.

Piecing Together Fossil Evidence

The partial skull generating all the excitement was found near the village of Bouri in Ethiopia in what is called the Middle Awash study area. Based on fossils discovered in earlier digs, hominids appear to have lived in the area for nearly six million years.

Proponents of the "bushy tree"/multiple-species view argue that African fossils dating to about two million years ago belong to Homo ergaster.Homo erectus, the thinking goes, split off about 1.6 million years ago, and existed only in Asia. The Asian branch was an evolutionary dead end, and the species Homo erectus died off.

Under this scenario, modern humans evolved from the original African branch of Homo ergaster.

The caves and volcanic soil of Africa are extremely conducive to fossil preservation, and scientists have been able to accurately date African fossils. Fossils found in Eurasia and Asia, however, are more difficult to date and until recently were thought to be much younger than those found in Africa. "Java man" of Indonesia, for instance, was originally placed in the 500,000-year-old range.